The present invention relates generally to piezoelectric devices and, more particularly, to an apparatus and method for accurately controlling movement of a piezoelectric device.
Piezoelectric devices alter their shape in response to an applied electric field. An electric field applied in the direction of polarization effects an expansion of the piezoelectric material in the same direction, while a voltage applied in the opposite direction of polarization will cause a contraction of the material in that same direction. In some applications, piezoelectric bending devices, such as thermally pre-stressed bending devices, use the xe2x80x9cbendingxe2x80x9d action of the device to convert electrical energy into mechanical energy. In such applications, the piezoelectric bending devices may be used as an actuator. In other applications, an outside force may impart a bending action to the device; and the device then converts that mechanical energy resulting from the outside force into electrical energy. In such applications, the piezoelectric bending devices may be used as a sensor.
In many applications, a voltage command signal is normally applied to the piezoelectric device to cause the device to move to a desired position. While such a control system functions well, there are nonlinearities in the operation of the piezoelectric device that are reflected back to the control system. For example, nonlinearities may cause the actuator to not fully move to a desired position in response to a voltage control signal. Further the control signal may cause the actuator to move or change its position in an abrupt or sluggish manner that does not match the desired actuator movement profile for a particular application.
Thus, the present invention is directed to overcoming one or more of the problems set forth above.
While the invention is described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
In accordance with the principles of the present invention, a control system is operatively connected to the piezoelectric device and responsive to a control signal representing a desired position of the piezoelectric device. The control system provides a command signal as a function of charge on the piezoelectric device that causes the piezoelectric device to move to the desired position. Thus, the present invention drives the piezoelectric device as a function of charge on the piezoelectric device. By utilizing charge as the controlling parameter, the control system of the present invention is free of some of the nonlinearities of a conventional control system that controls the piezoelectric device with a voltage command signal.
In accordance with one aspect of the present invention, the control system includes a charge control circuit responsive to the control signal for providing an output signal representing a desired change in charge on the piezoelectric device. The control system further includes a device control circuit responsive to the output signal of the charge control circuit for providing a command signal corresponding to the desired change in charge on the device.
In accordance with another aspect of the present invention, the charge control circuit includes an integrator circuit responsive to a detected current in the piezoelectric device and provides a charge feedback signal as a function of integrating the detected current over time. The charge control circuit further includes a comparator responsive to the first command signal and the charge feedback signal for providing a charge control signal to the piezoelectric device control circuit.
In another embodiment, the present invention provides a method of causing a piezoelectric device to move to a desired position by first determining a desired charge on the piezoelectric device. Next, a charge feedback is determined; and a control signal is applied to the piezoelectric device as a function of the desired feedback charges. The control signal causes the piezoelectric device to move to the desired position.
In accordance with one aspect of this embodiment of the invention, the charge feedback signal is determined by integrating a detected current flow through the piezoelectric device.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.